The present invention relates to electronic devices, and, more particularly, semiconductor devices useful in electronic communications on small computer system interface busses.
SCSI Bus
Communications between a computer system's central processor and peripheral devices typically occur over a parallel I/O bus. Tile Small Computer System Interface ("SCSI") bus is a parallel, multimaster I/O bus which has become a standard for processor communications with disk drives (both floppy and hard), tape drives, CD-ROMs, WORM drives, communications devices, and even bar code readers. The SCSI standard appears as ANSI X3.131-1986 and provides for 8-bit parallel data transfer with SCSI-2 proposing 16-bit and 32-bit capabilities at frequencies up to 10 MHz.
FIG. 1a schematically illustrates host computer 102 connected to peripherals 104 and 106 by the SCSI bus made of 50-conductor cables 114 and 116; cables 114 and 116 connect together inside of peripheral 104. The lines of a SCSI bus must be terminated, and cable 118 (which connects to cable 116 inside peripheral 106) leads to terminator 108. Host 102 may have an internal SCSI bus terminator for the other end of the bus. SCSI busses may be single-ended or differential; the signals on single-ended SCSI lines use a low close to ground and a high of about 3 volts, whereas the signals on differential SCSI lines use the polarity of the voltage difference between two lines with a difference magnitude minimum of 1 volt. A single-ended SCSI bus has 26 of the 50 lines of the 50-conductor cable tied to ground, whereas the differential SCSI bus grounds only 7 lines and has twice as many active lines as a single-ended SCSI bus due to its differential signal format. SCSI-2 proposes a 68-conductor cable to provide the extra data lines for 16-bit parallel data transfers.
In addition to the parallel data lines, a SCSI bus has lines for ground (GROUND), terminator power (TERMPWR), attention (ATN), busy (BSY), acknowledge (ACK), reset (RST), message (MSG), select (SEL), control/data (C/D), request (REQ) and input/output (I/O).
A SCSI bus includes a line in the 50-conductor cable to provide power (at about +5 volts) to the bus terminators; host 102 typically will power this line. FIG. 1b shows the single-ended passive terminator as consisting of a 220 ohm resistor in series with a 330 ohm resistor, so the lines will be biased to about +3 volts for a +5 volt terminator power (TERMPWR). The passive terminator for a differential SCSI has for each pair of lines a simple resistor string, as shown in FIG. 1c, connecting terminator power (TERMPWR) to ground with taps for the pair of lines (the RESERVED, GROUND, and TERMPWR lines are not terminated). Thus a +5 volt terminator power will bias the lines to about +2 and +3 volts for the - and + lines, respectively. Note that SCSI drivers are typically open collector, as indicated by FIG. 3. Thus drivers pull the lines low and released lines are pulled high by the terminator.
In contrast, the active terminator for single-ended SCSI-2 (also known as Alternative-2, Alt-2, or Boulay terminator) preferably supplies a 2.85 volts source in series with a 110 ohm resistor to each signal line (again, except for RESERVED, GROUND, and TERMPWR lines). Such a single-ended SCSI-2 active bus terminator will pull up the signal lines to 2.85 volts when the bus drivers of host 102 and peripherals 104 and 106 are off (in a high impedance state) and will supply current (about 22 mA) to cause a voltage drop of roughly 2.5 volts in the 110 ohm resistors when a bus driver on the line pulls it active low to about 0.5 volt. FIG. 2 shows a block diagram of the preferred single-ended SCSI terminator of ANSI X3. 131-1986.
The single-ended SCSI passive terminator has the problem of fluctuations of the voltage level in the power supply on line TERMPWR, injecting noise through the voltage reference and onto the signal lines; in addition, the Thevenin equivalent resistance 132 ohms does not match the cable characteristic impedance (90 to 140 ohms), thus causing signal reflections at higher frequencies and longer bus distances. And longer cable lengths will have greater transmission line effects, greater terminator power (TERMPWR) voltage drop giving lower pullup voltages in the passive terminator. And higher frequencies implies greater reflections from impedance mismatches, more noise injection, an so forth. However, active terminators can mitigate such problems at the cost of increased complexity. Consequently, there is a need for simple active terminators.